Lithographic printing presses use a so-called printing master such as a printing plate which is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-adhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the so-called computer-to-film method wherein various pre-press steps such as typeface selection, scanning, color separation, screening, trapping, layout and imposition are accomplished digitally and each color selection is transferred to graphic arts film using an image-setter. After processing, the film can be used as a mask for the exposure of an imaging material called plate precursor and after plate processing, a printing plate is obtained which can be used as a master.
A typical printing plate precursor for computer-to-film methods comprise a hydrophilic support and an image-recording layer of a photosensitive polymer layers which include UV-sensitive diazo compounds, dichromate-sensitized hydrophilic colloids and a large variety of synthetic photopolymers. Particularly diazo-sensitized systems are widely used. Upon image-wise exposure, typically by means of a film mask in a UV contact frame, the exposed image areas become insoluble and the unexposed areas remain soluble in an aqueous alkaline developer. The plate is then processed with the developer to remove the diazonium salt or diazo resin in the unexposed areas. So the exposed areas define the image areas (printing areas) of the printing master, and such printing plate precursors are therefore called ‘negative-working’. Also positive-working materials, wherein the exposed areas define the non-printing areas, are known, e.g. plates having a novolac/naphtoquinone-diazide coating which dissolves in the developer only at exposed areas.
In addition to the above photosensitive materials, also heat-sensitive printing plate precursors have become very popular. Such thermal materials offer the advantage of daylight-stability and are especially used in the so-called computer-to-plate method wherein the plate precursor is directly exposed, i.e. without the use of a film mask. The material is exposed to heat or to infrared light and the generated heat triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by cross-linking of a polymer, decomposition, or particle coagulation of a thermoplastic polymer latex.
WO97/39894 and EP-A 823 327 describe positive-working heat-sensitive materials comprising a hydrophilic support and a oleophilic coating provided thereon. The coating comprises a phenolic resin and a dissolution inhibitor, i.e. a compound which reduces the solubility of the phenolic resin in an aqueous alkaline developer. The interaction between the inhibitor and the phenolic resin is disrupted by exposure to heat or infrared light and, as a result, the exposed areas of the coating dissolve faster in the developer than the non-exposed areas and a lithographic image consisting of hydrophilic (exposed) and oleophilic (non-exposed) areas is obtained. In order to provide a larger solubility differentiation between exposed and non-exposed areas, WO 99/21725, EP-A 864 420 and EP-A 950 517 disclose the use of developer resistance means such as polysiloxane compounds which are capable of preventing the aqueous alkaline developer from penetrating into the phenolic resin layer. The increased developer resistance thus obtained can be reduced by exposure to heat or infrared light and upon subsequent immersion in the developer, a positive lithographic image is obtained.
The coating of the known printing plates contain a colorant, also called contrast dye or indicator dye, in order to provide a visible image after image-wise exposure and development. Such colorants remain in the coating at printing areas and are removed, together with the coating, at non-printing areas. Most of such prior art materials are characterized by a low sensitivity and therefore require a high power during exposure.